A safety system called Pre-Crash Safety System starts to come into practical use. This system determines beforehand a subject vehicle's state where a collision is unavoidable to thereby promptly activate safety equipment for decreasing collision damage (refer to Patent Document 1). In detail, a detector such as a millimeter wave radar detects a position, a distance, a speed, or the like of an obstruction ahead of a subject vehicle. When it is determined that collision with this obstruction is unavoidable, a seatbelt for a driver's seat or a driver-assistance's seat is fastened just before the collision so that fixation of an occupant is secured. Further, stepping of a braking pedal is detected to cause an enforced braking force. Avoidance of the collision or protection of occupants at the collision can be thereby achieved.                Patent Document 1: JP-2000-247211 A        
A conventional safety system such as the above works after a collision with an obstruction is determined to be unavoidable. Therefore, the system does not function for a purpose that a driver recognizes a danger beforehand to thereby conduct an operation for preventing the accident from occurring. A state where this safety system works is a last stage for collision avoidance or occupant protection. Activating this safety system may give an occupant discomfort or involve a rear-end collision with a following vehicle. It is therefore preferable that a driver conducts a proper operation before the above safety system is activated.
Analysis of causes of recent traffic accidents indicates that head-on collisions at intersections and rear-end collisions in straight roads account for the first and second largest portions, respectively. The above-described safety system detects danger of collision when a millimeter wave radar mounted in the front of a vehicle detects an obstruction. Therefore, the system only responds to an obstruction (a preceding vehicle, an on-road obstruction, or an oncoming vehicle) ahead of the subject vehicle, which is limited to uses in straight roads. In sum, the system does not function against head-on collisions at intersections.